Saturn and family from Cassini

A selection of images which featured in the wonderful talk by Professor Carl Murray (Queen Mary University, London) at the SPA Autumn 2010 meeting. Prof Murray's research interests are in solar system dynamics and currently the dynamics of Saturn's mysterious F-ring. Since 1990 he is working on the Imaging Science Subsystem on the Cassini spacecraft which is currently orbiting Saturn.

PIA 08329: Backlit Saturn and Earth

Looking back towards the inner solar system from Saturn (a distance of 1400 million kilometres), this image shows Saturn's disc and rings backlit by the Sun. It is a sobering thought that the tiny pale blue dot visible in this colour-enhanced image, is our planet Earth! (Look inside the G ring and above the brighter main rings to the left of Saturn.)

PIA 13424: The colourful equator of Rhea

These colour-enhanced images show recent bluish material which was presumably deposited on Rhea's surface since the formation of the impact craters. The image size is 130km across, about the size of the home counties of South-East England.

PIA 11688: Bursting at the Seams

A superb view of the plumes near the south pole of Enceladus, taken when Cassini flew through the jets on November 21st, 2009. The jets emerge from the tiger stripes which may contain liquid water, despite Enceladus receiving only 1 per-cent of the sunlight that Earth receives.

PIA 12481: Reflection of sunlight off Titan lake

As the Sun sets on Titan, sunlight glints off a hydrocarbon lake in this infra-red image taken on July 8th, 2009.

PIA 11690: Global view of Iapetus dichotomy

About 40 per-cent of the surface of Iapetus is coated in a dark material, deposited on the leading hemisphere. This material is thought to originate on Phoebe and causes the icy surface to melt further due to decreased reflectivity, relative to the uncoated trailing hemisphere. Over a long timescale this reinforces the contrast to its present-day level. The irregularly-shaped moon Hyperion is also coated but its chaotic rotation means the coating is uniform.

Shapiro delay weighs super-heavy neutron star

Radio astronomers in the U.S. and the Netherlands have announced the accurate weighing of a neutron star-white dwarf binary, revealing the neutron star to be almost 50 per-cent heavier than most other neutron stars. Their results appeared yesterday in the scientific journal Nature.

Neutron stars are found in a variety of binary stars, which provides a means to determine their masses using Kepler's Laws -- possibly modified to tske into account relativity.

A much more accurate technique using the Shapiro Delay (named after Harvard Professor Irwin Shapiro) is possible for the double neutron stars in which one star is a radio pulsar. If the path of the radio pulse passes close to the companion star it will be delayed by a tiny amount, which is measurable with very accurate clocks on Earth. The technique can also be applied to white dwarf-pulsar systems with reduced accuracy due to the weaker gravitational field of a white dwarf star.

As an example, the famous binary known as PSR B1913+16 is made up of neutron stars with 1.441 and 1.387 times the mass of our Sun. Surprisingly, these kinds of stars often turn out to have masses very close to 1.4 times the Sun's mass. The new observation of 1.97+/-0.04 times the Sun's mass for J1614-2230 is all the more remarkable, and provides new clues to physicists studying the interiors of neutron stars.

Double neutron star systems are also unique laboratories for testing General Relativity. The first one to be discovered, PSR1913+16 won Russell Hulse and Joseph Taylor the 1993 Nobel Prize in Physics. In 2004 an international team including scientists at Manchester University in the UK discovered an even rarer double pulsar.

Links

Astronomy Now article by Keith Cooper.
Ars Technica news story.

An Exoplanet Menagerie!

On 9th October Dr Peter Wheatley (Warwick University) gave a super and wide-ranging talk on the topic of Extrasolar Planets at the FAS convention in Cambridge. He mentioned the 10cm Schmidt camera STARE which inspired the SuperWASP camera on the Canary Islands, which now has 8 200mm f/1.8 lenses and has discovered 26 exoplanets to date. SuperWASP uses the transit method which allows astronomers to weigh the planets and sometimes to determine their radii too, which together constrain theories for their internal structure. One of the current unsolved mysteries is how some hot Jupiters are so enlarged for their mass, more than can be explained by the super-heating received as the planet hovers over its parent star.

Another remarkable discovery is the planet HAT-P-7b which is in a retrograde orbit according to observations of the Rossiter-McLaughlin effect. Although some moons in our solar system (notably Triton) orbit in the "wrong" direction, it's difficult to imagine how a planet can move against the flow of a rotating protoplanetary disk.

Dr Wheatley showed a chart showing all known exoplanets, grouped according to their size and the distance from their host star. I produced my own version below using data from the Extrasolar Planet Encyclopedia. It's remarkable that we know almost 500 exoplanets now; in 1999 I read "Planet Quest" by the renowned science writer Ken Croswell, at which time only 9 were known! With Kepler and SuperWASP-2 in development, the thousandth exoplanet discovery is just around the corner!

Links

The SuperWASP homepage

Postscript

I plotted Gliese 581g, a very recent discovery currently generating much excitement due to its position within the zone of habitability around its parent star, where liquid water and therefore Earth-like life may exist. It's the 6th planet found orbiting Gliese 581, a rather cool red dwarf star in Libra.